This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-393200, filed Dec. 25, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a magnetic random access memory and, more particularly, to a technique of improving the reliability of a line through which a large current flows in write operation.
As researchers have recently found that an MTJ (Magnetic Tunnel Junction) has a high MR (Magneto-Resistive) ratio at room temperature, the implementation of an MRAM to which a TMR (Tunneling Magneto-Resistive) effect is applied seems to be feasible.
Before the application of the TMR effect to an MRAM, an MRAM to which a GMR (Giant Magneto-Resistance) effect is applied has been known. However, the MR ratio of an MRAM to which the GMR effect is applied is several % to about 10%. In addition, in the MRAM to which the GMR effect is applied, a current flows in a thin metal film having a low resistance, as a result, the signal level is as low as several mV.
The MRAM to which the GMR effect is applied uses a technique of canceling out variations in characteristics among a plurality of magneto-resistance elements (memory cells) to prevent a data read error due to a very low signal level. Conventionally, for example, data read operation is performed twice with respect to a single memory cell to prevent the effect of variation in characteristics among magneto-resistance elements. It is therefore difficult to realize high-speed read operation in the MRAM to which the GMR effect is applied.
When a memory cell is comprised of a GMR element and a MOS transistor as a switch, if the ON resistance of the MOS transistor is not sufficiently low, a signal (cell data) read out from the memory cell may be lost due to the influence of variations in the characteristics of MOS transistor.
In order to prevent such a phenomenon, the ON resistance of the MOS transistor in the memory cell may be decreased to a value almost equal to that of the GMR element. To decrease the ON resistance of the MOS transistor in the memory cell to a value almost equal to that of the GMR element, however, the MOS transistor needs to have a considerably large size. This makes it difficult to attain a large memory capacity by decreasing the memory cell size.
As described above, in a GMR and MRAM, it is very difficult to realize high-speed memory operation and a large memory capacity. For this reason, the GMR and MRAM are used only under special environments, e.g., in space, owing to their characteristic feature, i.e., having excellent radiation resistance, but are not generally used much.
The basic structure of a TMR element is the MTJ structure in which an insulating film is sandwiched between two ferromagnetic layers. A magnetic member has a direction in which magnetization tends to be oriented, i.e., a magnetization easy axis. When a magnetic field is applied to a device in a specific direction during deposition of a ferromagnetic layer, the magnetization easy axis of each memory cell coincides with the specific direction.
The magnetization easy axis direction is the direction in which the internal energy in the magnetic layer is minimized when the direction coincides with the magnetization direction. If, therefore, no external magnetic field is applied, the magnetization of the ferromagnetic layer of the TMR element is oriented in the magnetization easy axis direction, and the relative directions of the magnetization of the two ferromagnetic layers are set in two different states, i.e., parallel and anti-parallel.
The TMR element changes in resistance depending on whether the magnetization directions of the two ferromagnetic layers are parallel or anti-parallel. It is generally assumed that this change is based on the spin dependence of tunneling probability.
As described above, binary data can be stored depending on whether the magnetization directions of the ferromagnetic layers of the TMR element are parallel or anti-parallel. In addition, cell data can be read out by using a change in the resistance of the TMR element due to a magnetization state.
The MR ratio of an MRAM using the TMR effect is several ten %, and a resistance value for the TMR element can be selected from a wide range of resistance values by changing the thickness of the insulating layer (tunnel insulating film) sandwiched between the two magnetic layers. In addition, in the MRAM using the TMR effect, the signal level in read operation may become equal to or more than the signal level in the DRAM.
In the MRAM using the TMR effect, write operation is performed by changing the magnetization direction of the TMR element (making it parallel or anti-parallel) using the magnetic field generated by currents flowing in two lines (line word line and bit line) perpendicular to each other.
More specifically, if the two ferromagnetic layers are made to have different thicknesses to set a difference between the coercive forces of the two magnetic layers, the relative directions of magnetization of the two ferromagnetic layers can be made parallel or anti-parallel by arbitrarily reversing only the magnetization of the thinner magnetic layer (having lower coercive force). In addition, if a diamagnetic layer is added to one of the two ferro-magnetic layers, and the magnetization direction of the magnetic layer to which the diamagnetic layer is added is fixed by exchange coupling, the relative directions of magnetization of the two ferromagnetic layers can be made parallel or anti-parallel by arbitrarily reversing only the magnetization of the magnetic layer to which the diamagnetic layer is not added.
A magnetic layer has the following property. Assume that the magnetization of the magnetic layer is to be reversed by applying a magnetic field in a direction opposite to the magnetization direction of the magnetic layer. In this case, if a magnetic field is applied in advance in a direction perpendicular to the magnetization direction, the magnitude of a magnetic field (reversing magnetic field) required to reverse the magnetization of the magnetic layer can be reduced.
By using two lines perpendicular to each other and applying magnetic fields in two directions perpendicular to each other, only the magnetization of the memory cell at the intersection of the lines can be selectively reversed.
FIG. 1 shows an asteroid curve.
The asteroid curve represents the magnitude of a magnetic field whose magnetization is reversed when a magnetic field parallel to the magnetization easy axis direction and a magnetic field perpendicular to the magnetization easy axis direction are applied at once.
In this case, the magnetization easy axis direction is the x direction.
Magnetization reversal does not occur unless the distal end of a magnetic field vector exceeds the asteroid curve. The three vectors shown in FIG. 1 represent the vectors of magnetic fields generated in the first memory cell area located at the intersection of two lines through which write currents flow and the second memory area adjacent to the first memory cell area.
If the magnitudes of currents flowing in two lines are controlled such that the distal ends of magnetic field vectors generated in the adjacent first and second memory cell areas fall within the asteroid curve, and the distal end of the resultant vector falls outside the asteroid curve, data can be selectively written in only the memory cell located at the intersection of the two lines in which the write currents flow.
A reversed magnetic field has the property of increasing in inverse proportion to the width of a magnetic member.
If, therefore, the memory cell size is decreased to increase the memory capacity, the width of the magnetic member decreases, and the reversed magnetic field must be increased. As a result, a larger current is required to generate a reversed magnetic field. On the other hand, with a reduction in memory cell size, the line width decreases, and hence the current density abruptly increases.
As the memory cell size decreases, an electromigration (EM) phenomenon tends to occur due to a large current required to generate a reversed magnetic field, resulting in a deterioration in line reliability.
If, for example, the aspect ratio of a line cross section is increased and the thickness of a line is increased to decrease the current density, the ratio of current components flowing far from a magnetic layer to the current flowing in the line increases. As a consequence, magnetic fields immediately below and above the line decrease in intensity. To compensate for this, a sufficiently large current must be supplied to the line. That is, this measure is not an effective means for preventing the occurrence of an electromigration phenomenon.
In addition, if the line thickness increases, the attenuation ratio of a magnetic field at a line adjacent to a line in which a large current flows decreases. This means that the interference of a write magnetic field with the adjacent cell (unselected cell) increases. That is, since reversed magnetic fields vary depending on memory cells, if the line thickness increases, the probability of write errors with respect to unselected cells increases.
As described above, according to the prior art, to prevent the occurrence of an electromigration phenomenon and improve the reliability of interconnections, for example, the use of the means of increasing the interconnection thickness has been considered. This means, however, cannot sufficiently decrease the current density of the interconnection. In order to prevent a write error with respect to an unselected cell, only the thickness of an interconnection is minimized, and the intensity distribution of the magnetic field generated by a large current flowing in the interconnection must be made as steep as possible with a minimum distribution width.
In other words, in the conventional MRAM, it is impossible to simultaneously satisfy the following requirements, an increase in memory capacity with a decrease in memory cell size, an improvement in the reliability of an interconnection, and prevention of write errors.
A magnetic random access memory according to an aspect of the present invention comprising a write word line, a bit line crossing the write word line, a magneto-resistance element which is placed at an intersection of the write word line and the bit line and stores data in accordance with a direction of magnetization that changes depending on a magnetic field generated by a current flowing in the write word line and a current flowing in the bit line, and a driver for causing the magneto-resistance element to store data by making a current flow in the write word line in a first direction, and then making a current flow in the write word line in a second direction.